The present invention relates to silicone emulsions. More particularly, the present invention relates to aqueous silicone emulsions that comprise a polyvinylalcohol component as an emulsifying agent.
Silicone emulsions comprising polyvinylalcohol, herein also referred to as PVA, of unspecified composition; or partially hydrolyzed polyvinylalcohol, herein also referred to as PHPVA, as an emulsifying agent have been known for a long time. For example, Dennett, U.S. Pat. No. 2,807,601, discloses a fabric treating composition comprising a hydroxy-endblocked polydimethylsiloxane fluid, a trimethyl-endblocked methylhydrogenpolysiloxane, PVA and sodium lauryl sulfate. As another example, Leavitt, U.S. Pat. No. 2,985,545, discloses a composition comprising a hydroxy-endblocked polydimethylsiloxane fluid, a methylhydrogenpolysiloxane, dibutyltin diacetate or dilaurate and a high viscosity PHPVA sold by E. I. duPont de Nemours Co. as Elvanol 50-42 and manufactured by partially (86-89%) hydrolyzing polyvinyl acetate. Leavitt also teaches that all nonionic emulsifiers, of which PVA is an example, are not satisfactory. Grenoble, U.S. Pat. No. 3,900,617, replaced the hydroxy-endblocked polydiorganosiloxane of Leavitt with a vinyl-endblocked polydiorganosiloxane and the tin salt with a platinum catalyst. Moeller, U.S. Pat. No. 4,008,346, prepared hydroxyl-endblocked polydiorganosiloxanes containing in-the-chain vinyl groups and vinyl-endblocked polydiorganosiloxanes by an emulsion polymerization process and used these emulsions, in conjunction with a platinum catalyst and an organohydrogenpolysiloxane, to prepare paper-coating emulsions. Moeller also added a dispersant to some of his coating emulsions, examples of which include a non-ionic, water-soluble hydroxyethyl cellulose and fully (98%) hydrolyzed polyvinylalcohol, herein also referred to as FHPVA. However, Moeller did not contemplate the use of 98% hydrolyzed polyvinylalcohol as an emulsifying agent for vinyl-containing polydiorganosiloxanes. Hockemeyer, U.K. Patent No. 1,541,801 claims a method for preparing an addition-curing coating emulsions which comprises mixing a curing catalyst with a vinyl polymer and thereafter emulsifying the mixture. Hockemeyer states that the emulsion can be formed with the aid of conventional dispersants, such as protective colloids, such as polyvinylalcohol having a degree of hydrolysis of 85 to 99 mol % and/or emulsifiers, such as polyglycol ethers of alcohols or alkylphenols. However, Hockemeyer apparently did not appreciate the benefit of using the more highly hydrolyzed polyvinylalcohol as an emulsifying agent for vinyl-containing polydiorganosiloxanes, since he disclosed no emulsions containing PVA. Traver et al., U.S. Pat. No. 4,190,688, prepared emulsions of hydroxyl-endblocked polydiorganosiloxanes containing in-the-chain vinyl groups and vinyl-endblocked polydiorganosiloxanes by the usual emulsifying methods, using PVA, and used these emulsions, in conjunction with a platinum catalyst or a tin catalyst and an organohydrogenpolysiloxane, to prepare paper-coating emulsions. As in Moeller, Traver et al, did not contemplate the use of FHPVA as an emulsifying agent for vinyl-containing polydiorganosiloxanes. Dallavia et al., U.S. Pat. Nos. 4,426,490 and 4,476,241, disclosed a catalyst emulsion comprising dioctyltin dineodeconate and PVA. Continuing the prevalent teachings of the past Dallavia et al. fail to specify the nature of the PVA and one can only conclude that patentees used the PHPVA that is occasionally specified in the art as a silicone emulsifying agent.
The silicone emulsion compositions of the art comprising PHPVA have been successful in the market place, principally because they do not contain significant amounts of solvents, the presence of which would require special handling to avoid toxic, safety and environmental problems, and because they do not require complex and/or expensive equipment for their preparation and use. However, PHPVA-based emulsions are not without drawbacks and improvements therein are being sought.
For example, PHPVA-based emulsions of curable silicone coating compositions typically lack the glossy appearance that characterizes their solventless and solvent-based analogs. Although the gloss of a silicone coating is regarded principally as an aesthetic quality it is, nevertheless, an important factor in the market place.
Additionally, PHPVA-based emulsions of adhesive-release coating compositions that are used to prepare peelable liners for pressure sensitive adhesives, herein also referred to as PSAs, sometimes display inadequate release of PSAs inasmuch as said release is too high. While the adhesive release force of a silicone coating can be increased by the addition of so-called high release additives, it is typically difficult to decrease its adhesive release force without compromising other properties of the coating, such as subsequent adhesion of the released adhesive, the curing profile of the coating or the film-integrity of the cured coating.
A coating having a glossy appearance and/or lower adhesive release can be obtained with an curable silicone emulsion composition, even though it has been prepared with only PHPVA, by decreasing the particle size of the emulsified curable silicone composition; however, this requires special handling and/or equipment, which adds to the cost of the composition.
Furthermore, the water resistance of a cured adhesive-release coating that results from a PHPVA-based silicone emulsion composition is, on occasion, insufficient to meet the processing requirements when an aqueous emulsion of a PSA is subsequently applied thereto.